1. Field of the Invention
The present invention relates to a digital watermarking system and a drift compensating system for enabling existence of copyright to be verified by watermarking a motion picture in delivering the motion picture and for enabling a source of a pirated edition to be decided by changing watermarked information in each destination.
2. Description of the Related Art
Most of the related arts are the systems of watermarking a baseband signal (non-compressed motion picture). For example, the system is disclosed in Japanese Patent Laid-Open Publication No. 2002-135737.
On the other hand, the system of directly watermarking a MPEG stream which is of the compressed motion picture is preferable because the amount of computing is largely decreased. However, a mismatch (drift) between encoded data and decoded data is generated, so that there is a problem that leads to degradation in image quality.
Therefore, in directly watermarking the MPEG stream, the drift compensating system has been proposed (F. Hartung and M. Kritter, “Multimedia Watermarking Technologies”, Proceedings of IEEE, Vol. 87, No. 7, pp. 1079-1107, July 1999).
In the drift compensating system described in the technical document, there is a problem that DCT coefficients of all image blocks are altered to watermark all the image blocks. In the method, because a motion compensating process is required for the drift compensation, there is another problem of the large amount of computing.